The present invention relates to a method of repairing reticles which are used for the production of semiconductor devices. In particular, the method relates to the use of vacuum ultraviolet (VUV) radiation, preferably 172 nm radiation, to remove defects resulting from ion beam repair of clear defects on reticles.
Reticles or masks are comprised of a patterned opaque coating such as chromium or chrome on a transparent substrate which could be quartz, glass, sapphire or other transparent materials. Reticles that are used to manufacture integrated circuits for semiconductor devices are approximately 6 inches by 6 inches square and are typically chrome on quartz. High quality reticles are necessary to provide maximum performance in the resulting device. One or more reticles are needed to build each of the several layers in a device.
Reticles are usually fabricated with expensive electron beam exposure tools that can take up to a day to write a pattern in a photoresist film on an underlying opaque coating. After the exposed photoresist film is developed in an organic or aqueous solvent to form a pattern, the pattern is transferred through the underlying opaque coating by an etch step. The photoresist serves as an etch mask and regions which are clear of photoresist before etch become regions clear of opaque coating after etch. The amount of opaque coating remaining after the etch transfer can vary from a few % to nearly all of the original surface area.
One of the objectives in making a high quality reticle is to maintain the size of the etched chrome feature within a tight tolerance. The tolerance is usually xc2x110% and preferably  less than 5% of a targeted dimension which is typically less than 1 micron for the smaller features in the pattern. The feature may be a line, trench, island or contact hole or a combination of two or more shapes with fixed or varying dimensions between the features. A second objective in reticle fabrication is to eliminate defects that are subsequently transferred to a device substrate in a patterning process and thereby cause reduced performance in the resulting integrated circuit. A third goal during the reticle making process is to minimize cost. Each mask or reticle can cost $20,000 or more and each product or device may require a set of over 20 reticles. Considering that a manufacturing line normally makes several products at once, the total number of reticles in a fab can easily be in the hundreds and can have a significant effect on the economic viability of the manufacturing line.
One of the most challenging tasks confronting reticle manufacturers is the removal of defects caused by pattern formation in the opaque film or resulting from repairs of other defects. Two common types of defects are opaque defects and clear defects. Opaque defects are regions of chrome that were intended to be clear of chrome. Clear defects are clear regions on the substrate that were intended to be covered with chrome. FIGS. 1a-1c show different types of clear defects for line/space and trench features and FIGS. 1d-1f show different types of clear defects on contact hole reticles. A fourth type of feature, islands, that are not represented in the drawings are small regions of chrome surrounded by a large area of clear substrate. A clear defect in this case is likely to be an island that is completely missing. Missing islands are repaired by the same methods as applied to clear defects in other features.
Two commonly used methods of reticle repair are laser ablation and focused ion beam (FIB) sputtering. FIB repair has become more popular since the ion beam can be more accurately focused than a laser beam and FIB causes less damage to the area surrounding the defect since it impinges the substrate with less energy than a laser. However, FIB sputtering impacts the substrate with enough energy to remove some substrate around the defect and its use should be minimized to avoid significant transmission loss through clear regions near the defect. U.S. Pat. Nos. 6,165,649 and 6,190,836 describe a method of forming a protective coating on a substrate to protect the chrome and quartz regions of a reticle before it is subjected to a laser or focused ion beam (FIB) repair of clear and opaque defects. An opening in a protective film is formed over the defect and then a FIB technique can be used to ablate unwanted chrome regions or to deposit an opaque material on unwanted clear regions. Once the repair is complete, the protective film is removed along with any particle defects that are deposited during the ablation or deposition step. This process is useful in preventing particle defects from repair techniques but requires extra steps of forming and removing a protective coating that will add to the cost of reticle fabrication. In the case of repairing clear defects, the opening in the protective coating requires an accurate control of the ion beam to form a shape that exactly uncovers the entire underlying clear defect but does not extend into an adjacent region that should remain clear of an opaque coating. As a result, an imperfect opening in the protective coating will lead to costly rework steps.
During FIB or laser repair of clear defects, the opaque material that is deposited outside the target area forms a peripheral film or halo which is thinner than the deposited film in the target area. The halo causes some transmission loss through the underlying quartz and reduces space width between two adjacent features. The quality of the reticle is degraded because the halo can cause a change in space width that is greater than the tolerance specified for the reticle. The halo is difficult to remove without damaging the reticle. An FIB technique can be employed to xe2x80x9ctrimxe2x80x9d the amount of halo but ion beam sputtering may remove some surrounding quartz as well which causes an undesirable nonuniform transmission across the reticle. In addition, FIB trimming can weaken the bonding between the deposit on the repair site and the underlying quartz. When carbon deposition is used to repair a clear defect, FIB trimming of the halo can lead to a lifting or peeling away of the carbon deposit during a subsequent cleaning step. Therefore, an improved method is needed for repairing clear defects, especially removing halos deposited during carbon deposition repair techniques that is less expensive than a protective coating and will not result in damage to the repair site or surrounding substrate during halo removal or in subsequent cleaning steps.
Besides the standard chrome on quartz reticle, more advanced xe2x80x9cphase shiftingxe2x80x9d reticles have been developed. A material is deposited next to a chrome region or in some cases the quartz adjacent to a chrome region may be etched to reduce its thickness. In either case, a differential in phase transmission is provided that enables features with smaller dimensions to be resolved in the patterning of photoresist films. The repair is complicated since a defect may occur in a phase shifted region of the substrate or in a region with no phase shift. In a phase shifted region, the defect could be an unwanted column of quartz that was not removed by etching or it could be a region where quartz was etched away to form a divot when it was intended to remain in place. U.S. Pat. No. 5,882,823 describes a method for repairing quartz divot and column defects in phase shift masks. The repair includes an FIB step and an isotropic etch but does not address the problem of removing halos resulting from FIB or laser repair of clear defects in chrome. Likewise, U.S. Pat. No. 6,103,430 describes a method for repairing divot and column defects in phase shift masks which comprises the steps of forming an opening in a photoresist coating over a defect, depositing and etching a spin on glass material to a level that is coplanar with the substrate, and removing the photoresist. Again, the method does not address the problem of clear defect repair in chrome regions which can occur on phase shift reticles.
Another problem associated with FIB repair methods is a charge build up on the substrate which is an insulator. The charge build up can result is an inaccurate placement of the beam away from the targeted repair area. U.S. Pat. No. 5,357,116 describes a conductive layer that prevents charging and is formed on the substrate prior to employing a prior art method of repairing a clear defect. The steps of coating a conductive layer and sputtering to form an opening in the layer prior to standard FIB repair lead to additional expense. Moreover, removal of the layer is performed with a flammable solvent that is a safety concern and can be costly to handle.
It will be an advantage of an improved method of clear defect repair to involve a minimum amount of process steps without additional coatings so rework can be reduced and the manufacturing cost can be kept at a minimum level.
An objective of the present invention is to provide a method for removing halos formed during a carbon deposition FIB repair of clear defects on a reticle such that there is no loss in transmission through the reticle. Furthermore, space width control should be maintained to within xc2x110% and preferably within xc2x15% of a targeted value in order to satisfy specifications. In addition, the carbon deposited at the target site should-not become distorted or lose adhesion during a subsequent cleaning process.
A further objective of the present invention is to provide a method of removing halos resulting from carbon deposition during clear defect repair that is cost effective and which can be implemented in a manufacturing line with minimal expense.
A further objective of the present invention is to provide an improved method of repairing a clear defect on a reticle.
These objectives are achieved by employing 172 nm VUV radiation which can be performed in a flood exposure mode in a commercially available tool. This process forms oxygen radicals in the air within the exposure chamber that react with carbon and hydrogen radicals generated from the carbon compounds in the halo on the reticle. Gaseous products of CO2 and H2O are formed that are swept away in the exhaust. This process avoids an ablation step that can damage surrounding substrate regions and change the transmission through the affected area. Microlithography simulation microscope (MSM) measurements are used to guarantee that the space width near the repaired area is within specification and that the repair site has not been damaged during a final cleaning step.
The method of using 172 nm VUV radiation according to the present invention is a cost advantage over prior art since it avoids the use of additional protective films and does not form additional particle defects which occur with ablative methods. A flood exposure tool to vaporize halos is less expensive than an extra FIB step to open protective coatings that require extra time for accurate placement of the beam over the defect. Also, the protective film can become charged during the FIB step to uncover the defect which results in an inaccurate placement of the beam that causes expensive rework.
The improved method of repairing a clear defect as shown in FIG. 2 is an advantage over previous art in that it is a high throughput process and does not damage the reticle. MSM measurements guarantee that space width between or within features is maintained within a tight tolerance and repair sites are not damaged during final cleaning steps. Rework is reduced with the present invention because it avoids extra FIB steps that require accurate placement to open protective films above the defect.
In a preferred embodiment, the present invention comprises the steps of (a) using a focused ion beam deposition of a carbon material in a clear defect region on a substrate, (b) exposing the entire substrate including a halo or peripheral film around a carbon deposit with 172 nm VUV radiation, (c) MSM measurements to compare space width in a region affected by carbon deposition to an unaffected region, (d) cleaning the substrate, and (e) MSM measurements to compare space width in a region previously covered with a halo to a region with no previous halo. The benefits of using this method are that the carbon halo is completely removed with no transmission loss through the reticle, space width is controlled to within 5% of a targeted value, and there is no lifting or peeling of the carbon deposit at the repair site during the cleaning steps.